To cope with the trend toward higher engine performance, lower fuel consumption, and the like, engine valves of some automobiles and the like are designed such that the engine valves are provided therein with a hollow portion in which sodium metal is filled, so as to achieve lighter weight and higher heat transfer.
In the case of such an engine valve, for example, first, a head portion is formed by forging or the like in a solid round rod-shaped raw material, and a hollow portion is formed by punching or the like inside a portion serving as a stem portion. Then, drawing is sequentially performed by inserting the stem portion sequentially into multiple, differently-sized cylindrical dies so that the outer diameter and inner diameter of the stem portion can be decreased in a stepped manner and the length of the stem portion can be increased in a stepped manner. As a result, a stem portion of a target size is formed (see Patent Literature 1 listed below and the like, for example).
Thereafter, (1) solid sodium metal is molded by extrusion into a rod shape, cut into a predetermined length, and inserted into the hollow portion, or (2) solid sodium metal is heated and melted (about 120 to 300° C.) to a liquid form and injected into the hollow portion in a predetermined quantity. After the sodium metal is thus put into the hollow portion, an opening at the upper end of the stem portion is sealed, whereby an engine valve in which sodium metal is filled is produced (see Patent Literature 2 listed below and the like, for example).